1. Field of the Invention
The present invention relates to a plasma treatment apparatus for accurately treating a workpiece such as a semiconductor wafer with plasma.
2. Description of the Related Art
In a semiconductor device manufacturing process using a plasma treatment apparatus that performs a plasma-etching treatment, a workpiece such as a semiconductor wafer to be treated with plasma should be kept at a predetermined position. For example, in the case of a planar plasma treatment apparatus that performs a plasma etching treatment in vacuum atmosphere, a workpiece is secured to a static chuck on a susceptor disposed on one of opposed electrodes.
A voltage is applied between a workpiece and an electrode which are opposed to each other with an insulation layer, positive and negative electric charges take place between the workpiece and the electrode and thereby Coulomb' force works therebetween. Thus, with the Coulomb' force, the static chuck attracts and holds the workpiece.
As such a static chuck, an insulating static attracting sheet may be disposed between a metal chuck and a workpiece. The static attracting sheet may be constructed by sandwiching a conductor such as copper with two sheets of insulating material such as polyimide.
In such a static chuck, when plasma generation is turned off after plasma treatment is completed, electric charges do not flow from the static chuck. Thus, a so-called float electrode is formed on the static chuck side. Consequently, electric charges stop flowing to the opposed electrode through plasma. As a result, the static chuck side is charged with residual electric charges. The electric charges on he static chuck side induce residual electric charges on the workpiece.
Before the workpiece is unloaded, the electric charges charged on the workpiece should be eliminated so as to easily unload the workpiece. As a conventional construction, conductive pusher pins which push up the workpiece and thereby detach it from the static chuck are provided. The pusher pins are made of a conductive material and are connected to a grounding circuit through a switch. In his construction, when the workpiece is pushed up, by turning on the switch, the electric charges on the workpiece can be eliminated.
However, when the electric charges on the workpiece are eliminated, the following problem takes place.
When the switch is turned on and off while the pusher pins are being pushed up, noise tends to take place due to discharging of the electric charges. In other words, when the switch is turned off, since the pusher pins are electrically kept in a floating state, if the density of electric charges charged on the workpiece is high, electric charges are discharged. In addition, when a grounding circuit, which is connected to the pusher pins, is provided, the construction becomes complicated.
To simplify the construction, a circuit which directly grounds a resistor or a coil instead of the above-mentioned switch may be constructed.
However, in such a construction, noise cannot be eliminated. The inventor of the present invention has found that noise is induced by a potential between the pusher pins and the susceptor.
In other words, a circuit that supplies an RF power to the susceptor is provided with for example a condenser that prevents a DC current from flowing to the power supply. In this case, the condenser or storage capacitance of the power supply circuit itself serves as an insulator in DC. When the supply of the RF power is stopped, electric charges that can sufficiently charge the susceptor may remain in the condenser or the circuit. Thus, when the pusher pins are grounded, a potential tends to take place between the pusher pin side and the susceptor. Thus, an abnormal discharging will take place, which is a cause of noise.
When residual charges remain on the susceptor side, even if the power to the static chuck is turned off, the workpiece is attracted to the susceptor side by a static induction. Thus, the workpiece is not easily peeled off from the static chuck. If the workpiece is attempted to be forcefully peeled off, it will be damaged.